Traction control systems of motor vehicles limit wheel slipping in which a drive wheel overruns its traction surface. Slipping occurs when more torque is imparted to a drive wheel than can be withstood by its traction surface for correspondingly moving the vehicle. The excess torque causes a sudden increase in drive wheel rotational speed with respect to its traction surface, referred to herein as wheel slipping.
Traction measured as a force is a function of wheel slip measured as a percentage of overall drive wheel rotation that is in excess of rolling contact with the traction surface. A small percentage of wheel slip is needed to fully exploit the available traction force, but larger percentages of wheel slip reduce the traction force. Accordingly, wheel slipping, i.e., large percentages of wheel slip, actually reduces the amount of power that can be used to move a vehicle. Excess wheel slip also reduces lateral stability.
Many traction control systems have evolved as extensions of anti-lock braking systems. However, instead of reducing brake pressures in response to wheel skidding, the traction control systems increase brake pressures in response to excessive wheel slip. The same sensors can be used by both systems to monitor rotational speeds of the wheels.
However, the use of individual drive wheel brakes for traction control has many disadvantages. For example, the application of individual drive wheel brakes can produce shocks in the drive line or reflect excess torque between paired drive wheels resulting in drive line instabilities known as "hunting". Excessive use of the brakes causes accelerated wear. Engine output power can often overwhelm the power-absorbing capacities of the wheel brakes. Also, the application of the wheel brakes requires the generation of fluid pressure and its controlled conversion into mechanical braking torques, which can delay appropriate braking responses.
Other traction control systems regulate engine output power to limit wheel slip. The engine output power of internal combustion engines is controlled by regulating ignition, air intake, fuel intake, or exhaust. Engine controllers already regulate some or all of these functions, so little additional hardware is required for traction control. However, throttle controls are sometimes preferred for directly overriding operator commands to the engine.
Although most engine output controls, including throttle controls, have nearly unlimited capacity for reducing output power to the drive wheels, the response to excessive wheel slip is slow. For example, significant wheel slipping and associated further loss of traction can occur before output power can be sufficiently reduced to regain traction. Overcompensation for wheel slip can also limit vehicle acceleration, uphill speeds, and towing capacity, which detract from potential vehicle performance.
Some hybrid traction control systems combine engine output power controls with drive wheel brake controls for limiting wheel slipping. However, the combination does not necessarily mitigate the drawbacks of using engine output power controls or wheel brake controls separately. For example, primary use of the wheel brakes can still cause drive line shocks and accelerated wear, whereas primary use of the engine output power controls is still too slow to prevent excessive wheel slip.
U.S. Pat. No. 5,303,794 to Hrovat et al. discloses another hybrid traction control system, which combines engine output power controls with specially controlled clutches of a multiple clutch transmission for regulating output power to a pair of drive wheels. Each of the clutches connects the engine to the drive wheels at a different speed ratio. Partially engaging one of the clutches to the so-called "higher gear" speed ratio while another clutch is already engaged causes a windup in the drive train which further loads the engine. The windup generates some additional friction because of higher loading forces and the engagement of more gears, but the amount of friction must be limited to avoid over-stressing the gears. The main effect on the engine is believed to come from transferring torque to the drive wheels through the partially engaged clutch at a lower mechanical advantage, similar to the immediate effect of upshifting the transmission.
However, if the transmission is already in high gear, the engine load cannot be further increased by at least partially engaging both clutches. In fact, the partial downshifting engagement of a lower gear would decrease engine load by improving mechanical advantage of the engine over the drive wheels. Conversely,the amount of additional engine load by partially upshifting is limited by the difference between the engaged and partially engaged speed ratios. Also, engine inertia effective through backlash in the drive train would cause the drive wheels to momentarily increase in speed, which delays the desired effect of limiting wheel spin.